Polyethylene modified asphalt compositions

ABSTRACT

A polymer modified bituminous composition for roofing applications that comprises polyethylene as the primary modifier.

TECHNICAL FIELD OF THE INVENTION

This invention relates to bituminous roofing. More particularly, thepresent invention relates to modified asphalt composition for use inroofing materials.

BACKGROUND OF THE INVENTION

It is well known to use asphalt compositions to manufacturewaterproofing materials, such as roofing membranes, shingles orunderlayments. The purpose of such materials is to offer protectionagainst the elements. Because of its good weatherability and hydrophobicnature, asphalt has been used in waterproofing applications for severalcenturies.

Roofing materials must possess key mechanical and chemicalcharacteristics, such as pliability, resistance to cracking at lowtemperatures, high puncture resistance, good strength and elongation (towithstand stresses and potential building movements), resistance to flowat elevated temperatures, and thermal and dimensional stability, etc.Most importantly, a roofing material must have minimal susceptibility tothe effects of temperature. It is also desirable that such properties bemaintained even after exposure to natural elements, e.g. sunlight,condensation, airborne contaminants, etc., and foot traffic.

It is well known that asphalt by itself does not possess all theseessential properties. For example, asphalt undergoes stiffening at lowtemperatures, which makes it fragile in cold weather temperatures evento light impacts. At high temperatures, instead, the viscosity ofasphalt is so low that it flows spontaneously or becomes permanentlydeformed under minimal

mechanical stress. To overcome these problems, modifiers have been addedto the asphalt in order to obtain an asphalt composition having moresuitable physical and mechanical properties. Polymers commonly used tomodify asphalt include amorphous or atactic polypropylene (APP),amorphous polyalphaolefin (APAO), thermoplastic polyolefin (TPO),styrene-butadiene-styrene (SBS), styrene-ethylene-butadiene-styrene(SEBS), synthetic rubber or other asphaltic modifiers that enhance theproperties of asphalt. The incorporation of such modifiers into theasphalt widens its workable temperature range for roofing applicationsand results in improved mechanical and viscoelastic properties. Some ofthe main reasons for modifying asphalt with polymers are to obtaingreater resistance to flow at high temperatures, improved resistance tocracking at low temperatures, high dimensional stability over a widetemperature range, improved durability, and resistance to weatheringcaused by exposure to the elements.

In the middle of the 20^(th) century, an innovative roofing systemutilizing modified asphalt was introduced in the market. This type ofroofing system is generally referred to as “modified bituminousroofing,” or “modified asphalt roofing membrane” and eliminates the needfor several layers of roofing sheets to be installed on the rooftop.Such roofing materials are comprised of a core that is saturated andcoated with modified asphalt. The core is typically a reinforcingcarrier made of fabric such as polyester, fiberglass, or a combinationof both. The asphalt modifiers typically include atactic polypropylene(APP), amorphous polyalphaolefin (APAO), thermoplastic polyolefin (TPO),styrene-butadiene-styrene (SBS), styrene-ethylene-butadiene-styrene(SEBS), synthetic rubber or other asphaltic modifiers that enhance theproperties of asphalt.

The modified bituminous roofing materials described above are used incommercial, industrial and residential applications. Two majorclassifications of such materials, that are used mostly in industrialand commercial applications are (1) cap sheet and (2) base sheet. Thecap sheet takes its name from the fact that its top surface is exposedto the elements. Because their top surface is also exposed to theelements, shingle roofing materials, which are used primarily inresidential applications, can also be considered “cap” materials. Bothcap sheets and shingles can be manufactured using asphalt modified withAPP, APAO, TPO, SBS, SEBS, synthetic rubber or other asphalticmodifiers, and are generally reinforced with a polyester carrier or acombination of polyester and fiberglass. They can be smooth or granularsurfaced and are typically greater than 2.8 mm in thickness. Base sheetsare typically modified using any of the same modifiers as a cap sheet.Because the base sheet is not intended to be exposed to the elements,the asphalt component is typically modified using less expensivepolymers such as APP, or with smaller quantities of polymers such asSBS. The base sheet is typically reinforced with a fiberglass carrier,which is significantly less expensive than polyester. It is smoothsurfaced and typically between 1.0 mm to 2.5 mm thick depending upon thejob specifications.

In a typical field installation, a base sheet is first applied to theroof deck using mechanical fasteners, hot mopping or cold applicationtechniques. Cap sheets or shingles are applied on top of the basesheets, with the seams of adjacent rolls in offset relation.Underlayments, which are typically reinforced with fiberglass, but canalso have no carrier are more commonly used under shingles. Most APPmodified asphaltic membranes are torch applied by heating the back sideof the sheet to melt the compound and using the molten compound to forma heat weld. Most SBS modified asphaltic membranes are set by hotmopping, torch-application or adhesion using cold-process adhesives asdescribed in U.S. Pat. No. 5,807,911, to Wentz, et al.

SBS is one of the most commonly used asphalt modifiers. It is amade-on-purpose polymer that is widely used in several applications suchas in the manufacture of shoes. SBS-modified asphaltic blends haveimproved low temperature flexibility, pliability and elongationproperties. However, blends containing SBS as the only modifier alsohave disadvantages. For example, they tend to be soft and thus difficultto install during warm weather. They have poor resistance to ultravioletlight from the sun, and thus need to be protected with a coating ormineral granules on the upper surface. Furthermore, SBS's uniquemolecular structure makes processing SBS polymer difficult, andconsequently manufacturing an SBS-asphalt compound requires specialmixing equipment.

APP is another commonly used modifier in asphalt blends. It is aby-product in the production of isotactic or crystalline polypropylene(IPP). Because of its low cost and excellent characteristics inmodifying asphalt, it has been used for asphalt modification for overforty years. APP modified asphalt blends have higher resistance atelevated temperatures than SBS modified asphalt blends, improved lowtemperature properties, good heat aging and long-term weatherabilitycharacteristics, and excellent walkability on warm days. APP modifiedasphalt compounds also possess better ultraviolet resistance than theirSBS counterparts. Asphalt modified with only APP, however, does not haveenough hardness, and is susceptible to deformation under minimalmechanical force, especially at high temperatures. Thus when modifyingasphalt, APP modifiers are usually used in conjunction with otheringredients such as ethylene-propylene copolymer and IPP. Addition ofIPP to an APP-asphalt blend imparts hardness and rigidity to the blend.Incorporation of ethylene-propylene copolymer improves its lowtemperature flexibility. Depending upon the type of asphalt and thedesired final properties, varying proportions of the different modifiersare added to asphalt.

Of the two general types of asphaltic sheet materials used for roofingapplications, i.e., APP modified or SBS modified, the SBS based productsare more elastic, and have greater flexibility at low temperatures. APPbased products, on the other hand, are more resistant to heat (due to ahigher softening point), atmospheric effects (especially ultra-violetrays), and foot traffic.

The cost of polymeric modifiers is the highest of all ingredients in theasphalt formulation and therefore has a significant effect on the totalcost of the formulation. In order to reduce costs, attempts have beenmade to include cheaper materials as modifiers, for example,polyethylene. Attempts to use polyethylene as a modifier, however, havebeen limited to using low density polyethylene as the primary modifieror using it in conjunction with other modifiers such as APP, IPP andethylene propylene copolymer in the formulation. The primary purpose ofsuch attempts was to replace more expensive and scarcer raw materialssuch as APP. While APP was originally the unwanted by-product ofcrystalline polypropylene synthesis, it is becoming increasingly scarcebecause processes for making IPP have been improved to the point whereproduction of by-product APP is negligible. Despite these attempts andthe need to find lower cost materials, to the best of the applicants'knowledge, no modified asphalt roofing membranes composed of asphaltmodified with polyethylene as the primary modifier have yet beenmarketed. There is therefore a need for a polyethylene-based modifiedasphalt composition which has properties that are ideally suited formodified asphalt roofing systems.

SUMMARY OF THE INVENTION

The present invention provides novel asphalt compositions for use inroofing applications that utilize polyethylene (virgin or recycled) asthe primary or sole asphalt modifier. It has been discovered that byusing a blend of low and high density polyethylene and/or a plasticiser,ethylene modified asphalt formulations can be obtained which havesurprisingly good characteristics for modified asphalt roofing membraneapplications. The formulations of the present invention surprisinglyhave improved mechanical, chemical and rheological properties comparedto asphalts modified with conventionally used polymers such as APP,APAO, or SBS. They are also characterized by enhanced processabilityduring manufacture as well as ease of application during fieldinstallation. In particular the formulations of the present inventionpossess high temperature resistance, good long-term weatherabilitycharacteristics, and excellent durability. Furthermore the formulationsof the present invention can be formulated so as to be very “hard”, andthus ideally suited for application in warm weather conditions, or very“flexible”, and thus, ideally suited for application in cold weatherconditions. The formulations are also advantageous because they offersignificant cost savings in comparison to the typical APP, APAO or SBSmodified asphalt. The ethylene is not only cheaper than these othermodifiers, but also surprisingly less can be used in the formulation ofthe invention. This leads to enhanced cost savings. In the preferredembodiments, 10% or less of polymer can be used, whereas in membranesmodified with conventional polymeric modifiers, the polymer content istypically 15% or more.

Further aspects, objects, features and advantages of the presentinvention will become apparent to those of ordinary skill in the artupon reading and understanding the following detailed description andpreferred embodiments, and when read in light of the accompanyingexamples.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

As discussed in the earlier sections, the widely employed approach tomodify the physical, chemical and rheological properties of asphalt foruse in roofing materials is to incorporate certain modifiers, includingAPP, APAO, TPO, SBS, SEBS, and synthetic rubber. Of these, the mostpopular are APP, and SBS.

The present invention is a novel asphalt formulation for use inmanufacturing roofing membranes which includes polyethylene as theprincipal modifier. Polyethylene-asphalt roofing blends are notcommercially available because compatibility between polyethylene andasphalt is notoriously difficult to achieve. Compatibility betweenasphalt and polymers is important to ensure stability of the blend andobtain the desired performance. In fact, SBS must be used with certaintypes of asphalt to avoid phase separation. However, whereas SBS-basedblends may have improved low temperature flexibility and elasticity,they do not have as good UV resistance as APP-modified blends.

It has now been surprisingly found that the use of low densitypolyethylene (“LDPE”) and high density polyethylene (“HDPE”), in definedratios ranging from 4:1 to 1:4 LDPE/HDPE by weight, depending on theapplication, as the principal polymeric modifier in an asphaltcomposition results in properties not only equal to, but also superiorto modified asphalt compositions of the prior art. The PE basedcompositions of the invention not only have excellent low temperatureflexibility and elasticity, but also good UV resistance and higherresistance at elevated temperatures. It has further surprisingly beenfound that the addition of a plastification agent, such as an aromaticoil, to a polyethylene-asphalt blend improves the phase compatibilitybetween asphalt and polyethylene so that a commercially useful blendwith excellent characteristics for roofing results. Such blends exhibitgood performance at low temperatures as well as high temperatureconditions and are therefore well suited for roofing applications. Inparticular, the polyethylene modified asphalt compositions of thepresent invention have improved long-term properties, such as improvedlow temperature flexibility and elasticity after aging, and offersignificant cost savings compared to the conventional APP or SBS asphaltformulations of the prior art.

In one embodiment of the invention, the composition comprises betweenabout 1 and 20 percent by weight of polyethylene, about 5 and 50 percentby weight of filler, 1 to 10 percent by weight of one or moreplastification agents, and the remaining portion asphalt. Preferably thepolyethylene-asphalt composition of this embodiment includes between 5and 15 percent by weight of polyethylene, between 5 and 50 percent byweight of filler, between 3 and 6 percent by weight of plastificationagents, and between 40 and 70 percent by weight of asphalt. Mostpreferably, the polyethylene-asphalt blend of this embodiment comprises5 to 10 percent by weight of polyethylene, 20 to 35 percent by weight offiller, 4 to 5 percent by weight of plastification agents, and 50 to 65percent by weight of asphalt. Fire retardant materials can be used asfillers in order to produce a polyethylene-asphalt blend having fireretardant properties.

In another embodiment, the polyethylene comprises high densitypolyethylene and low density polyethylene in a ratio that ranges fromabout 4:1 to 1:4 LDPE/HDPE by weight, depending on the desired hardnessof the composition. The harder the desired composition, the higher theHDPE component. The asphalt composition of this embodiment optionallyfurther includes one or more plastification agents. Preferably thepolyethylene-asphalt composition of this embodiment includes betweenabout 1 and 20 percent by weight of polyethylene, about 5 and 50 percentby weight of filler, optionally about 1 to 10 percent of aplastification agent, and the remaining portion asphalt. If aplastification agent is used, the composition of the present inventionpreferably comprises between about 5 and 15 percent by weight ofpolyethylene, between about 5 and 50 percent by weight of filler,between about 3 and 6 percent by weight of plastification agents, andbetween about 40 and 70 percent by weight of asphalt. Most preferably,the polyethylene-asphalt blend of this embodiment comprises about 5 to10 percent by weight of polyethylene, about 20 to 35 percent by weightof filler, about 4 to 5 percent by weight of plastification agent, andabout 50 to 65 percent by weight of asphalt. Fire retardant materialscan be used as fillers in order to produce a polyethylene-asphalt blendhaving fire retardant properties. A particularly preferred roofingasphalt formulation includes about 10 percent by weight of polyethylene,about 4 percent by weight of plastification agents, about 31 percent byweight of filler, and about 55 percent by weight of asphalt.

Asphalts are usually classified according to their geographic origins,such as Arabian, Venezuelan, Canadian, Rocky Mountain, Mid-Continentcrude, and their properties vary based on the crude source. Asphalt canbe either a product of refining of crude oil or a naturally occurringsubstance, asphalt used in this formulation is obtained during theprocess of distillation of petroleum. The asphalt preferably used in themanufacture of roofing shingles is normally “oxidized”. Oxidation isdone by the introduction of air, i.e. oxygen, into asphalt and heatingthe same, and this process of oxidation makes the asphalt hard. Suchhard asphalt is preferred for the manufacture of roofing shingles andunderlayments, and may contain filler. However asphalt to be used in themembranes of the present invention is preferably not oxidized and doesnot contain any filler.

The characteristics of the modified asphalt blend vary dramaticallydepending upon the source of crude from which the particular type ofasphalt is derived. Hence it is essential to analyze the chemistry ofthe asphalt to determine saturates and asphaltene levels. Asphaltpreferably used in the formulations of the present invention has theproperties described in Table I, below. TABLE I PHYSICAL PROPERTIES OFASPHALT Properties Unit Values Viscosity @ 180 degrees C. cP 30-300Boiling point ° C. >350 Softening point ° C.  >35 Flash point ° C. >475Needle penetration at 60 deg C., 100 g/5 sec dmm 50-200Preferred asphalt for this application is referred to in the art as“roofing flux”. Commonly used asphalts of this type are RF-140, PG52-28, PG 70-22 (where “RF” refers to “roofing flux”, “PG” refers to“pavement grade”, the numerical designation “140” refers to the targetneedle penetration value, and “52-28” indicates the workable temperaturerange of the asphalt). The asphalt employed in the present invention maybe obtained from various refineries. Some of the sources of asphalt usedin this invention are Valero Refining Company of San Antonio, Tex.,United Refining Company of Erie, Pa., and Foreland Refining Corporationof Salt Lake City, Utah.

The polyethylene used in formulations of the present invention may below density polyethylene (LDPE), high density polyethylene (HDPE), andpreferably a combination thereof. Linear polyethylenes are preferred.Linear polyethylenes are formed in the presence of a catalyst and haveminimal branching of carbon chains having more than ten carbon atoms.

The preferred polyethylene for use in the present polyethylene modifiedasphalt blends has a weight average molecular weight of between 1,000and 25,000, a melt flow index value between about 5 and 100, preferablyabout 5-50 and most preferably about 18-22, and can come in the form ofpowder, granules or bales. Polyethylene may be low, medium or highdensity, as characterized in the Table II below. TABLE II POLYETHYLENEDENSITIES Density Type Unit Density Low Density Polyethylene g/cm³ lessthan 0.915 Medium Density Polyethylene g/cm³ 0.915-0.950 High DensityPolyethylene g/cm³ greater than 0.950Preferred low density polyethylene is less than about 0.950 g/cm³,preferably less than 0.930 and most preferably less than about 0.915g/cm³ Preferred high density polyethylene most preferably has a meltflow index of about 2 to 20. Polyethylene employed in thepolyethylene-modified asphalt blends of the present invention may bevirgin or recycled material. Of course the use of recycled materialoffers significant cost savings. Whereas virgin polyethylene may beprocured from a variety of manufacturers of polyethylene, recycledpolyethylene may be obtained from companies that specialize in recycledplastics. A list of such companies may be obtained from localgovernmental agencies in charge of recycling programs. A few sources ofrecycled plastics are Ravago Plastics of Houston, Tex., The Matrixx Co.of Houston, Tex., First State Recycling Company of Wilmington, Del.,Agri-Plas of Portland, Oreg., and SJC Limited of Fernley, Nev.

Where a mixture of low density and high density polyethylene is used ina formulation of the invention, the preferred ratio between thedifferent density polyethylene components depends upon the chemistry ofthe base asphalt and the desired characteristics of the resultant blend.For example low density polyethylene (LDPE) tends to lower softeningpoint, but imparts greater flexibility to the blend, whereas highdensity polyethylene (HDPE) tends to increase the softening point.Addition of high levels of high density polyethylene (HDPE) also affectsthe flexibility or elasticity of the mix, owing to its crystallinemorphology. Depending upon the desired results, which vary from summerto winter months, varying proportions of the two polyolefins may beemployed. Thus, the formulations may be adjusted to account for seasonaltemperature fluctuations simply by adjusting the ratio between the highand low density polyethylene. Furthermore the amount of plasticiser maybe varied or the plasticiser may be omitted altogether. Thus, forexample, in order to get a “hard” compound, which is more suitable forroofing materials to be installed at warm temperatures, the ratio ofhigh to low density polyethylene is increased. In order to get a “soft”compound, which is more suitable for roofing materials to be installedat cold temperatures, the ratio of high to low density polyethylene isdecreased. The preferred ratio of LDPE to HDPE for a blend used in amembrane to be applied in warm weather is 1:1. The preferred ratio ofLDPE to HDPE for a blend used in a membrane to be applied in coldweather is 4:1. The higher the ratio of HDPE to LDPE, the greater theamount of plasticiser needed, if used. Furthermore, because theplasticiser tends to reduce hardness, it is advantageous to omit it fromformulations used in roofing materials to be installed at warmtemperatures.

Greater or lesser amounts of filler may be used; though the quantity offiller used affects the viscosity and the final mix cost. The fillerused in the embodiments enumerated above is selected from the groupconsisting of limestone, talc, fly ash, volcanic ash, graphite, carbonblack, silica, stone dust or china clay. Addition of filler material hasnumerous advantages such as reducing the overall cost of theasphalt-polymer blend, improving fire resistant properties, andincreasing the resistance to weather. Additionally, in order to achievefire ratings as classified by Underwriters' Laboratories (UL), specialfire retardant additives may be used as filler material. Typical fireretardants employed include calcium borate, magnesium borate, a mixtureof antimony tri-oxide and deca bromo diphenyl oxide, etc. These are usedas replacement for existing filler material such as limestone, talc, flyash, volcanic ash, graphite, carbon black, silica, stone dust or chinaclay or in conjunction with these materials. A minimum of 10 percent ofthe fire retardant material is preferred to achieve the desiredperformance during fire testing.

Plastification agents are preferably aromatic oils that are availablefrom oil refineries around the country. It is preferable that such oilhas a level of ‘aromatic’ content sufficient to achieve goodcompatibility with asphalt. Preferably the plastification agent is alinear or branched alkylphenol, most preferably the oil is a linearalkylphenol. Such oils are commercially available. The plastificationagent may be added separately to asphalt (before other modifiers areadded) or alternately it may be added at the time of preparation of theasphalt-polymer blend.

Asphalt-polymer blends may be prepared in any number of ways known tothose skilled in the art. Some of the methods of mixing include Banburymixers, screw extruders, auger mixers, etc. During the process ofmixing, the various ingredients are mixed with asphalt at temperatureshigher than the melting point of the additives such that the polymerswill melt and is said to form a cross-linked network with asphalt. It isthis cross-linked network that imparts the desired characteristics toasphalt. Mixing times can vary from 3 hours to 6 hours, depending uponthe size and shape of the additives, mixing temperature, mixing speed,method of addition of the resins, etc. The conditions at whichpolyethylene is blended with asphalt is important and affects the finalproperties. Typically, mixing of the polyethylene in asphalt is done atambient pressure and at a temperature between 150° C. and 240° C.Preferably temperature range of mixing is between 180° C. and 210° C.Use of polyethylene in liquid form improves the efficiency of the mixingprocess. A laboratory batch may be prepared in a steel container, bymixing the right quantities of the various ingredients with asphalt attemperature between 180° C. and 210° C., and by mixing using a bladestirrer at speeds varying from 400 rpm to 1000 rpm for 3 hours to 6hours. A master batch may be prepared by incorporating all theingredients in an external mixer and mixing at temperatures ranging from180° C. and 210° C. for 4 hours to 6 hours.

EXAMPLE I

Examples of preferred embodiments of the inventive formulation appear inTable III below. The LDPE used in these examples has a density of lessthan 0.93 g/cm³ and the HDPE used in these examples has a melt flowindex of 7. TABLE III Warm Cold Ingredients Unit application blendapplication blend Low density polyethylene % 5.0 6.0 High densitypolyethylene % 5.0 1.5 Plastification Agent % 0 2.5 Filler % 35.0 35.0Asphalt % 55.0 55.0The formulation is prepared by loading all the ingredients in anexternal mixer and mixing the resultant mixture at temperatures rangingfrom 180° C. and 210° C. for 4 hours to 6 hours. The resultantformulation should have a viscosity between 2,000 and 20,000 cP at 180°C., a softening point temperature greater than 120° C., and a needlepenetration value of greater than 100 dmm at 60° C.

Various tests were conducted on the polyethylene modified asphalticblends described in Table II using ASTM test methods and standardscommonly used in the art to determine suitability for roofingapplications. The results are summarized in Table IV, below. Anexplanation of the properties measured follows. TABLE IV Warm Coldapplication application Properties Unit blend blend Phase compatibilityvisual very good very good Viscosity @ 180° C. cP 7000 4500 Softeningpoint ° C. 125 120 Penetration @ 60 degrees C. dmm 70 120 Lowtemperature flexibility ° C. −5 −5 Heat stability ° C. 120 120 Low temp.flexibility after heat aging ° C. +5 −5 Cost of mix per lb US$ 0.0530.0725

Phase compatibility is an essential property because it is an indicationof the long-term performance of the blend. Phase compatibility, alsoreferred to as “networking”, affects other key mix characteristics suchas viscosity, softening point, penetration, low temperature flexibility,heat stability and long term heat aging. Phase compatibility is usuallyassessed by taking small samples of the blend, forming it into a thinfilm on a microscope slide and observing the dispersion of the polymersin the asphalt under a microscope at 250 magnification. This visualobservation under the microscope is usually compared to a chartcontaining photographs of films showing gradations of phasecompatibility, ranging from, for example, good to poor. Typically eachroofing manufacturer has its own chart, however what is considered goodand poor is generally consistent within the industry.

Viscosity is a measure of the flow characteristics of a material.Viscosity of the modified asphaltic blend is essential to ensure ease ofprocessing during the process of saturating and coating thereinforcement with the asphaltic blend. A viscosity that is too high mayresult in stretching and cause wrinkles on the reinforcing fabric. Aviscosity that is too low may make it difficult to achieve the desiredthickness of the final membrane because the blend may flow readily.Viscosity is also a factor during field installation of the roofingmembrane. For example, in the case of torch-applied membranes, it may bedifficult to achieve a good flow of asphalt from membrane manufacturedusing a high viscosity blend. Conversely membranes consisting of lowviscosity blends may flow too readily and may be too “soft” to walk onduring roof installation. Hence it is important that an optimumviscosity be maintained. Optimal viscosity ranges depend on thematerials and the processing conditions at the production facility.However, those of ordinary skill in the art are very familiar withoptimal viscosity ranges associated with different blends for particularuses and types of roofing materials. Generally, however, the viscositymust be between 2,000 and 20,000 cps. The most commonly used measure ofviscosity is known as “Brookfield viscosity”, which is obtained using aBrookfield viscosometer. It is a measure of the flow characteristics ofa material and is expressed in centipoise. The viscosity is determinedusually at the temperature at which the blend will be processed in themanufacturing line.

Softening point is the temperature at which the blend reaches a state atwhich it begins to flow. This parameter is important factor during fieldinstallation of the roofing membrane. For example, in the case oftorch-applied membranes, if the blend used to manufacture the membranehas a softening point that is too high, it may be difficult to achieve agood flow of asphalt during installation. Conversely, if the blend usedto manufacture the membrane has a softening point which is too low, itmay flow too readily and may be too “soft” to walk on. Hence it isimportant that the blend have a an optimum softening point for thetemperature range during which the membrane will be installed. Thus thisparameter must be adjusted to accommodate different seasons or climates.Softening point is determined by the “ring and ball” test method inaccordance with ASTM D-36, and is expressed in ° C. Optimal softeningpoint ranges depend on the materials and the processing conditions atthe particular plant. However, those of ordinary skill in the art arefamiliar with optimal softening point ranges associated with differentblends for particular uses and types of roofing materials. Generally,the softening point must be between about 110 and 160° C.

Needle penetration is an indication of the softness or hardness of thematerial. It is also an important factor during field installation ofthe roofing membrane. For example, a mix that is too soft may causeproblems associated with “treading” and result in footprints being lefton the membrane surface in warm climates. A blend that is too hard maycrack in cold climates. This parameter is also adjusted by those ofordinary skill in the art to accommodate diverse climates and theseasons. This test is conducted in accordance with ASTM D-5 test method,where a sample is conditioned in a water bath maintained at 60° C.subjected to needle penetration under a weight of 100 grams for fiveseconds. The depth the needle travels in those five seconds is used asthe measure of hardness or softness of the blend. It is expressed in dmm(decimillimeter). The optimal needle penetration ranges depend on thematerials and the applications (e.g. how the roofer uses the product).However, those of ordinary skill in the art are familiar with optimalneedle penetration ranges associated with different blends forparticular uses and types of roofing materials. Generally, the needlepenetration at 60° C. should be between 40 and 150 dmm.

Low temperature flexibility is a measure of the ability of the blend toresist cracking at low temperatures. This parameter is very importantbecause it is a measure of how well and for how long the roofingmembrane will perform. It is very important that membranes have good lowtemperature flexibility, especially during cold weather conditions. Todetermine low temperature flexibility, samples of the blend areconditioned in a freezer at a constant temperature for 1 to 2 hours, andthen bent over a mandrel of a particular diameter over a time period ofusually 5 seconds. The sample is then visually observed for signs ofcracking. If the sample does not exhibit any signs of cracking, testingis continued at lower temperatures until signs of cracking are visible.Low temperature flexibility is also determined after heat conditioningthe samples in a convection oven maintained at 70 degrees C. for 90 daysin order to determine the low temperature flexibility after aging orheat aged cold flexibility. This parameter is important for membranesthat will be subjected to extremes in temperature.

Heat stability is a measure of the dimensional stability of the blend athigh temperatures. It is directly related to the softening point. Wherethe softening point is measured on the blend, the heat stability ismeasured on the final product. Dimensional stability is a measure of howwell the membrane retains its dimension and shape. This test isconducted by vertically hanging samples of the blend in a convectionoven at different temperatures and observing at what temperature anydripping or flowing of asphalt occurs.

COMPARATIVE EXAMPLE

The present invention will be further understood and appreciated byreference to the following example of conventional APP modified asphaltblend formulations. The composition of the blends is described in TableV. They are prepared in the same method as the polyethylene-modifiedasphalt blends described in Table III. TABLE V Warm Cold IngredientsUnit application blend application blend Atactic polypropylene % 5.0 6.0Isotactic polypropylene % 5.0 2.0 Copolymer % 5.0 7.0 Filler % 35.0 35.0Asphalt % 50.0 50.0

The compositions shown in Table V are typical APP modified asphalticformulations used in roofing applications. APP used is commerciallyavailable material such as Adflex from Basell Company. IPP employed is arecycled material from First State Recycling Company of Wilmington, Del.Copolymer utilized was an ethylene-propylene copolymer, Vestoplast 891,obtained from Degussa GmbH of Germany. Such a mix typically has gooddispersion, a viscosity of 2,000 to 10,000 cP at 180° C., softeningpoint greater than 120° C., and needle penetration value of greater than50 dmm at 60° C. Results of typical parameters are given in Table VI.TABLE VI Warm Cold application application Properties Unit blend blendPhase compatibility visual very good very good Viscosity @ 180° C. cP4500 3000 Softening point ° C. 140 140 Penetration @ 60 degrees C. dmm50 120 Low temperature flexibility ° C. −5 −10 Heat stability ° C. 140140 Low temp. flexibility after heat aging ° C. +10 +5 Cost of mix perlb US$ 0.099 0.113

The data in Tables IV and VI shows that the formulations of theinvention are comparable to the conventional APP modified formulationsin many respects. The heat stability and softening point of the blendsof the invention, while lower than those of the conventional blends, aremore than adequate for most roofing purposes. Furthermore, theformulations of the present invention are a significant improvement overcomparable conventional polypropylene formulations with respect to lowtemperature flexibility after heat aging. As discussed above, this is avery important parameter because it assesses the durability and longterm performance of the membrane. As can be seen from the data, the warmapplication blend of the invention and the corresponding polypropyleneblend had a low temperature flexibility of −5° C. After heat aging, thewarm application blend of the invention had a low temperatureflexibility of +5° C., whereas the corresponding value for theconventional blend was +10° C. Furthermore, while the low temperatureflexibility of the cold application blend of the invention, before heataging, was −5° C. versus −10° C. for the corresponding value for theconventional APP cold application blend, the cold application blend ofthe invention after heat aging had a low temperature flexibility of −5°C., which is ten degrees lower than the corresponding value observed forthe conventional APP cold application blend.

In addition, the formulations of the present invention providesignificant cost savings due both to the lower cost and lower quantityof the polymer. As can be seen when comparing Tables IV and VI, the costof the warm application blend of the invention is about 45% lower thanthe cost of the corresponding conventional blend, and the cost of thecold application blend of the invention is about 35% lower than the costof the corresponding conventional blend. Furthermore, the blends of theinvention also offer greater ease of processing because fewer resinsmust be added to the asphalt.

1. A modified bitumen composition for roofing application comprising: apolymer modifier consisting essentially of polyethylene; and asphalt,wherein the polyethylene consists of a mixture of high densitypolyethylene and low density polyethylene in a ratio ranging from about4:1 to 1:4 LDPE to HDPE by weight.
 2. The composition of claim 1,wherein the polyethylene is linear.
 3. The composition of claim 1,wherein the polyethylene has a molecular weight between about 1,000 and25,000.
 4. The composition of claim 1, wherein the polyethylene has amelt flow index value between about 5 and
 50. 5. The composition ofclaim 1, wherein the low density polyethylene has a density less than0.95 g/cm³.
 6. The composition of claim 1, wherein the low densitypolyethylene has a density less than 0.93 g/cm³.
 7. The composition ofclaim 1, wherein the low density polyethylene has a density ranging lessthan about about 0.915 g/cm³ and the high density polyethylene has amelt flow index ranging from 2 to
 20. 8. The composition of claim 1,further comprising at least one plastification agent.
 9. The compositionof claim 8, comprising: a. about 1 to 20 percent by weight ofpolyethylene, b. about 1 to 10 percent by weight of a plastificationagent, c. about 5 to 50 percent by weight of filler, and d. remainingportion of asphalt.
 10. The composition of claim 9, comprising: a.between about 5 to 15 percent by weight of polyethylene, b. betweenabout 3 to 6 percent by weight of plastification agents, c. between 5and 50 percent by weight of filler, and d. between 40 and 70 percent byweight of asphalt.
 11. The composition of claim 10, comprising: a. about5 to 10 percent by weight of polyethylene, b. about 20 to 35 percent byweight of filler, c. about 4 to 5 percent by weight of plastificationagent, and d. about 50 to 65 percent by weight of asphalt.
 12. Thecomposition of claim 11, comprising: a. about 10 percent by weight ofpolyethylene, b. about 4 percent by weight of plastification agent. 13.The composition of claim 1, comprising: a. about 5 percent by weight oflow density polyethylene; b. about 5 percent by weight of high densitypolyethylene.
 14. The composition of claim 8, comprising: a. about 6percent by weight of low density polyethylene; b. about 1.5 percent byweight of high density polyethylene; c. about 2.5 percent by weight ofplastification agent.
 15. A modified bitumen composition for roofingapplication comprising: a polymer modifier consisting essentially ofpolyethylene; one or more plastification agents; and asphalt.
 16. Thecomposition of claim 15, wherein the polyethylene consists of a mixtureof high density polyethylene and low density polyethylene in a ratioranging from 1:4 to 4:1 LDPE to HDPE by weight.
 17. The composition ofclaim 15, wherein the polyethylene is linear.
 18. The composition ofclaim 15, wherein the polyethylene has a molecular weight between 1,000and 25,000.
 19. The composition of claim 15, wherein the polyethylenehas a melt flow index value between 5 and
 50. 20. The composition ofclaim 15, wherein the low density polyethylene has a density less than0.95 g/cm³.
 21. The composition of claim 15, wherein the low densitypolyethylene has a density less than 0.93 g/cm³.
 22. The composition ofclaim 15, wherein the low density polyethylene has a density rangingless than about about 0.915 g/cm³ and the high density polyethylene hasa melt flow index ranging from 2 to
 20. 23. The composition of claim 15,wherein the low density polyethylene has a density ranging less than0.91 and the high density polyethylene has a melt flow index rangingfrom 2 to
 20. 24. The composition of claim 15, comprising: a. about 7.5percent by weight of polyethylene, and b. about 2.5 percent by weight ofplastification agent.